feat(D.2b): UiChatScrollbar — track/thumb/buttons driving UiScrollable

Implements the right-side chat scrollbar widget. Ports retail
UIElement_Scrollbar::UpdateLayout @0x4710d0 (thumb sizing + placement)
and HandleButtonClick @0x470e90 (step ±1 line, page on track click).

Dat element ids sourced from chat LayoutDesc 0x21000006 (base layout
0x2100003E): up-button sprite 0x06004C69, down-button 0x06004C6C, track
0x06004C5F, thumb middle 0x06004C63. Up/down buttons occupy the top and
bottom ButtonH (16px) regions of the widget height, matching element
positions Y=0 and Y=32 in the base scrollbar template.

Adds 6 pure ThumbRect tests (no GL): sizing, clamping to MinThumb,
position at start/mid/end, no-overflow full-fill.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/AcDream.App.Tests/UI/UiChatScrollbarTests.cs

@@ -0,0 +1,81 @@
+using AcDream.App.UI;
+using Xunit;
+
+namespace AcDream.App.Tests.UI;
+
+/// <summary>
+/// Pure unit tests for <see cref="UiChatScrollbar.ThumbRect"/> — no GL dependency.
+/// </summary>
+public class UiChatScrollbarTests
+{
+    // Model: content=400, view=100, trackLen=200.
+    // ThumbRatio = 100/400 = 0.25  →  thumbH = max(8, 200*0.25) = 50.
+    // Travel = 200 - 50 = 150.
+
+    [Fact]
+    public void ThumbRect_AtStart_HasCorrectSizeAndZeroOffset()
+    {
+        var m = new UiScrollable { ContentHeight = 400, ViewHeight = 100 };
+        // PositionRatio = 0 (start).
+        var (y, h) = UiChatScrollbar.ThumbRect(m, trackTop: 0f, trackLen: 200f);
+        Assert.Equal(50f, h, 3f);
+        Assert.Equal(0f, y, 3f);
+    }
+
+    [Fact]
+    public void ThumbRect_AtEnd_PinsToBottomOfTrack()
+    {
+        var m = new UiScrollable { ContentHeight = 400, ViewHeight = 100 };
+        m.ScrollToEnd(); // PositionRatio = 1.
+        float trackTop = 16f, trackLen = 200f;
+        var (y, h) = UiChatScrollbar.ThumbRect(m, trackTop, trackLen);
+        Assert.Equal(50f, h, 3f);
+        // y = trackTop + travel * 1 = 16 + 150 = 166.
+        Assert.Equal(166f, y, 3f);
+    }
+
+    [Fact]
+    public void ThumbRect_WithButtonH_CorrectlyOffsetsFromTrackTop()
+    {
+        // Matches task spec: content=400, view=100, trackLen=200, PositionRatio=1.
+        // thumbH=50; travel=150; y = trackTop + 150 = trackTop + 150.
+        var m = new UiScrollable { ContentHeight = 400, ViewHeight = 100 };
+        m.ScrollToEnd();
+        var (y, h) = UiChatScrollbar.ThumbRect(m, trackTop: 16f, trackLen: 200f);
+        Assert.Equal(50f, h, 3f);
+        Assert.Equal(166f, y, 3f); // 16 + 150
+    }
+
+    [Fact]
+    public void ThumbRect_MidScroll_InterpolatesPosition()
+    {
+        // content=400 view=100 → MaxScroll=300; ScrollY=150 → PositionRatio=0.5.
+        var m = new UiScrollable { ContentHeight = 400, ViewHeight = 100 };
+        m.SetScrollY(150);
+        Assert.Equal(0.5f, m.PositionRatio, 3);
+
+        var (y, h) = UiChatScrollbar.ThumbRect(m, trackTop: 0f, trackLen: 200f);
+        Assert.Equal(50f, h, 3f);
+        // y = 0 + 150 * 0.5 = 75.
+        Assert.Equal(75f, y, 3f);
+    }
+
+    [Fact]
+    public void ThumbRect_SmallContent_EnforcesMinThumb()
+    {
+        // content=1000, view=10, trackLen=200 → ThumbRatio=0.01 → raw=2 < 8 → clamp to 8.
+        var m = new UiScrollable { ContentHeight = 1000, ViewHeight = 10 };
+        var (_, h) = UiChatScrollbar.ThumbRect(m, trackTop: 0f, trackLen: 200f);
+        Assert.Equal(8f, h, 3f);
+    }
+
+    [Fact]
+    public void ThumbRect_NoOverflow_ThumbFillsTrack()
+    {
+        // content <= view → ThumbRatio = 1 → thumbH = trackLen.
+        var m = new UiScrollable { ContentHeight = 50, ViewHeight = 100 };
+        var (y, h) = UiChatScrollbar.ThumbRect(m, trackTop: 16f, trackLen: 100f);
+        Assert.Equal(100f, h, 3f);
+        Assert.Equal(16f, y, 3f); // travel = 0 → y = trackTop
+    }
+}